Many liquids from aqueous solution to liquid metals such as steel may contain impurities which end up in the solid and affect the purpose for which they are used. For example, aqueous solutions as by-products from mineral dressing processes may contain a residue of fine particles in excess of that allowed. The effluent from a sewage works, for example, may be in excess of such levels thought to be safe for drinking purposes.
In many of these aqueous processes a simple filtration process can be used in order to obtain a cleaner residue. In other cases, such as those with high temperatures, for example aluminum melts, it is possible to use relatively coarse forms of filter material. These take the form of beds of alumina balls graded in size from top to bottom with filter depths on the order of 100-150 centimeters. These filters are capable of filtering out the entrained oxide created by turbulent metal transfer and also the fine but very hard particles such as the TiC (titanium carbide) derived from the electrolytic recovery process. However, problems exist in that filters only last until they are saturated with oxides and other particles and when this occurs, the rate of metal flow through them decreases. Moreover, any movement of the filter can send quantities of extraneous material down a launder and contaminate a large quantity of metal.
Magnesium is also prone to pick up this form of oxide contamination and in the case of aluminum bronze such contaminates are an accepted hazard.
In the case of ferrous alloys, the contaminants are derived from re-oxidation during metal transfer and from the addition of `killing` elements such as aluminum and silicon used to counteract this re-oxidation of the melt. The by-products of these reactions are oxides of silicon and aluminum. The former will be in the form of complex, liquid silicates which form long stringers in the final products and in the latter case hard crystalline alumina particles, which again cause problems during casting, such as blockage of the nozzles through which the steel is conveyed from the ladle to the tundish. Particles also aggregate into large clusters which have a deleterious effect on metal properties.
It is therefore an object of the present invention to obviate or mitigate at least some of the above-mentioned disadvantages.